In order to monitor the performance of wavelength division multiplexed (WDM) optical networks it is useful to have a measurement of the signal to noise ratio (SNR) of each wavelength channel at various points throughout the network. In networks that are predominantly degraded by amplified spontaneous emission (ASE) noise the SNR is correlated with the optical signal to noise ratio (OSNR).
The OSNR has traditionally been measured with an optical spectrum analyser (OSA). The in-band OSNR can be estimated by interpolating ASE noise floor measured at points between adjacent wavelength channels. However, the OSA method fails for WDM systems with high spectral efficiency, as shown in FIG. 1, in which the optical spectrum of 10 GB/s NRZ signals with 50 GHz spacing (0.1 nm resolution) is illustrated. Notably, the modulation sidebands between such closely spaced channels mask the true OSNR level, and reduction of the resolution bandwidth of the OSA will be of no benefit. The OSA method also fails in reconfigurable networks where different channels may traverse through different optical paths.
There have been a number of alternative proposals to directly measure the in-band OSNR (i.e., the OSNR within the signal bandwidth), which attempt to overcome the above limitations. These in-band methods use a variety of techniques to distinguish the signal from the noise.
One such proposed method of in-band OSNR measurement is the asynchronous histogram technique set out in “Application of Amplitude Histograms to Monitor Performance of Optical Channels,” Elec. Lett., vol 35, pp 403, March 1999. In this proposal, a high speed receiver is used to build up an asynchronously sampled histogram. While a simple concept, this method has difficulty in distinguishing various sources of noise impairment, and further requires a high speed detector.
A polarisation nulling method of in-band OSNR measurement is set out in J. H. Lee, D. K. Jung, C. H. Kim and Y. C. Chung, “OSNR Monitoring Technique Using Polarisation-Nulling Method. IEEE Phot. Tech. Lett., vol. 13, pp 88-90, January 2001 and in United States Patent Application No. 2001/0052981. In this method the degree of polarisation of the channel is correlated with the OSNR. Using a quarter wave plate the signal is transformed into a linear state of polarisation and passed through a polariser. The OSNR is obtained from the ratio of the maximum and minimum average optical powers as the polariser is rotated.
A narrowband RF analysis at half-clock frequency is proposed in H. Stuart, “Signal to Noise Ratio Monitoring of Optical Data Using Narrowband RF Analysis at the Half-Clock Frequency,” OFC 2003, pp 407-409. This technique is based on the assumption that the Fourier transform of a return to zero (RZ) electrical signal at half the clock rate is real. SNR is therefore said to be able to be determined from in-phase and quadrature measurements of electrical signal at half clock rate. This method is inherently narrowband and so requires minimal high speed electronics, and further is not limited to ASE noise. However, this method is dependent on signal format and bit rate, and can only measure RF noise at half bit rate for RZ signals. Further, this method assumes that noise measured at the half bit rate is indicative of noise across signal bandwidth, and assumes that noise sources are isotropic in phase space.
An orthogonal delayed homodyne technique is set out in C. J. Youn, K. J. Park, J. H. Lee and Y. C. Chung, “OSNR Monitoring Technique Based on Orthogonal Delayed-Homodyne Method,” IEEE Phot. Tech. Lett., vol. 14, pp 1469-1471, October 2002 and in United States Patent Application No. 2004/0126108 A1 The delayed homodyne technique relies on the perpendicular polarisation components of the ASE field being uncorrelated. FIG. 2 illustrates this principle of the delayed homodyne technique. The signal field is aligned at 45° to the axes of a polarisation beam splitter (PBS) so that the signal amplitudes in two axes are identical (s1(t)=s2(t)). In contrast, although the symmetric nature of the ASE noise ensures that there is equal noise power in both arms, the ASE amplitudes in the two arms of the PBS, a1(t) and a2(t), are uncorrelated.
A schematic of the experimental setup of the delayed homodyne technique for measuring in-band OSNR is shown in FIG. 3. The WDM channel is separated into the two arms of a polarisation beam splitter (PBS) with a delay of about 400 ps in one arm and then recombined with a second PBS. The module within the dashed square is thus simply a first order PMD emulator. The RF spectrum of the signal output has a null at a frequency determined by the time delay (fnull=½Δτ). In contrast, the ASE noise spectrum is not affected by the delay since the ASE fields in the different arms of the PBS are uncorrelated. A measure of the radio frequency (RF) spectral power at the null thus gives a direct measure of the ASE noise. The delayed homodyne technique works with depolarized or circularly polarised light, the depth of the null is insensitive to first order polarisation mode dispersion (PMD) and the frequency of the null is dependent on differential group delay (DGD). The delayed homodyne technique requires equal powers in both arms of the PBS, necessitating a polarisation controller (PC) at the PBS input to compensate for fluctuations in the input state of polarisation. Further, for a fixed time delay, the null only exists at one frequency, and measurement of the OSNR can only occur at that frequency.
A RF spectral null analysis technique with polarisation maintaining (PM) fiber has been proposed in G. W. Lu, M. H. Cheung, L. K. Chen and C. K. Chan, “Simultaneous PMD and OSNR Monitoring by Enhanced RF Spectral Dip Analysis Assisted with a Local Large-DGD Element,” ECOC 04, such technique being based on a similar principle to Balanced Homodyne technique. However in this case, the relative delay in the polarisation states is obtained by introducing a polarisation maintaining (PM) fiber with a large DGD in place of the PMD emulator.
United States Patent Application No. 2002/0149814 discloses a multi-function optical performance monitor. This document is based on a 4 port PBS with the outputs incident upon two photo detectors [6]. The input of the PBS consists of the signal port and a local oscillator (mixing port). The monitor is designed to measure signal impairments such as PMD and dispersion.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.
Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.